1. Technical Field of the Invention
The present invention relates to an organoaluminum compound obtained by reacting an organoaluminumoxy compound with hydroxy compound. The present invention further relates to use of the organoaluminum compound as a catalyst. The present invention further relates to a method for producing isopulegol by cyclizing citronellal using the organoaluminum compound as a catalyst.
2. Background of the Invention
Conventionally, menthol, particularly, l-menthol, is very important as flavor or fragrance having fresh-feeling and its use is very wide. As the synthesis method of menthol, a method of obtaining dl-menthol by optical resolution and a method of obtaining menthol by an asymmetric synthesis method are known (Non-Patent Document 1). In the production step of l-menthol by the asymmetric synthesis method, l-menthol is obtained by hydrogenating l-isopulegol which is a precursor. However, in order to synthesize the l-isopulegol, selective cyclization reaction of d-citronellal is an important step.
As the selective cyclization reaction of d-citronellal, the disclosed method, that is, the production of l-isopulegol using zinc bromide as a catalyst, was already performed (Patent Document 1). In this case, the ratio between l-isopulegol and other isomers is about 90% as diastereoselectivity.
Selective cyclization reaction by an aluminum siloxide catalyst is reported (Patent Documents 2 and 3). In this case, the diastereoselectivity is up to 96%.
Selective cyclization reactions by tris(2,6-diarylphenoxy)aluminum and its similar catalyst are reported (Patent Documents 4 to 8). In those Patent Documents, the diastereoselectivity of isopulegol formed is about 96%.
On the other hand, regarding an aluminum catalyst having a diol skeleton which is a ligand derived from tartaric acid, there are many reports (Patent Document 9) (Non-Patent Documents 2 to 5). However, those reports each are concerned with only an aluminum catalyst having a cationic complex or a specific substituent such as halogen group and aminohydroxy group.